The invention relates to a method for monitoring an SCR catalytic converter, in which an NH3 storage capacity of the SCR catalytic converter is monitored.
Methods and devices for operating an internal combustion engine, in particular in a motor vehicle, in the exhaust-gas region of which is arranged an SCR catalytic converter (Selective Catalytic Reduction) by means of which the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine are reduced in the presence of a reducing agent to form nitrogen, are known. The proportion of nitrogen oxides in the exhaust gas can be considerably reduced in this way. For the reaction to take place, ammonia (NH3) is required, which ammonia is admixed to the exhaust gas. As reactant or reducing agent, therefore, use is made of NH3 or reagents which split off NH3. In general, use is made of an aqueous urea solution which is injected into the exhaust section upstream of the SCR catalytic converter by means of a dosing device. NH3, which acts as reducing agent, is formed from said solution. The dosing of the reducing agent takes place preferably as a function of the nitrogen oxide emissions of the engine, and is therefore dependent in particular on the present speed and torque of the engine.
The basic principle of the SCR catalytic converter is that of the nitrogen oxide molecules being reduced on the catalytic converter surface in the presence of ammonia to form elementary nitrogen. The required dosing rate is determined in an electronic control unit, in which are stored strategies for the operation and monitoring of the SCR system.
Within the context of the so-called on-board diagnosis (OBD), the SCR system must be monitored as a component which is relevant with regard to emissions. Here, the OBD limit values, which are specified usually as a multiple of the legally defined emissions limit values, must be adhered to. With regard to the SCR catalytic converter, it must be ensured that the OBD limit value for nitrogen oxides is adhered to, that is to say the monitoring functions must ensure that, in the event of the corresponding limit value being exceeded, the SCR catalytic converter is identified as being defective.
To measure nitrogen oxide values, an SCR catalytic converter generally comprises at least one NOx sensor. Presently conventional NOx sensors have cross-sensitivity to NH3, such that the sensor signal indicates not exclusively the NOx concentration but rather a summed signal of NOx and NH3. The sensor signal from a NOx sensor arranged downstream of an SCR catalytic converter therefore cannot be assigned unequivocally to the NOx concentration or the NH3 concentration. That is to say, an increase of the sensor signal downstream of the SCR catalytic converter can on the one hand be a sign of an increase in NOx concentration and therefore a decreasing NOx conversion rate in the catalytic converter. On the other hand, an increase in the signal may also indicate a breakthrough of pure ammonia, that is to say an increase in NH3 concentration. A direct distinction is not possible between NOx and NH3. Since ammonia is harmful to health and to the environment in high concentrations, the breakthrough of pure ammonia, so-called NH3 slippage, should as far as possible be prevented.
Of decisive importance for the functioning of the SCR catalytic converter is its storage capacity for NH3. If the NH3 storage capacity is reduced on account of aging of or damage to the catalytic converter, an increased breakthrough or slippage of NH3 may occur, and the conversion of the nitrogen oxides no longer takes place to an adequate extent. Methods for monitoring an SCR catalytic converter are already known in which the storage capacity for NH3 is taken into consideration as a measure of the aging of or damage to the catalytic converter. For example, the laid-open specification DE 10 2007 040 439 A1 describes a method for diagnosing an SCR catalytic converter, downstream of which is connected a nitrogen oxide sensor with cross-sensitivity to ammonia. The SCR catalytic converter is initially filled in an overdosing phase with a superstoichiometric reducing agent dosing up to the maximum NH3 storage capacity, in order to attain a defined starting point for a diagnosis. The attainment of the maximum NH3 storage is detected, on the basis of the breakthrough of pure ammonia downstream of the SCR catalytic converter, as NH3 slippage. The NH3 slippage can be indirectly measured on account of the cross-sensitivity of the nitrogen oxide sensor to NH3. Subsequently, in an underdosing phase with a reducing agent dosing which is reduced in relation to a normal dosing or is absent, the stored NH3 mass is gradually depleted again by the reduction of the nitrogen oxides in the catalytic converter. During said emptying test, the usable NH3 storage capacity can be indirectly determined on the basis of one or more characteristic values which is/are dependent on the NOx conversion rate, because with a lower stored NH3 mass, less NOx can be converted on the catalytic converter surface during the emptying test.
It is important for the described monitoring method that the NH3 slippage is detected in good time in order to be able to switch from the overdosing phase into the underdosing phase and avoid an unnecessary release of ammonia. To detect the NH3 slippage, use has hitherto been made of predefinable thresholds for the SCR efficiency or for the NOx sensor signal downstream of the SCR catalytic converter, which are evaluated under certain monitoring conditions. Here, NH3 slippage is detected if, during the overdosing phase, after a predefinable time, the sensor signal increases above a predefinable threshold or the measured SCR efficiency falls below a predefinable threshold. A problem with defining thresholds is however that, in the case of a threshold with only a small deviation from the normal state, the slippage detection is highly sensitive and there is the risk that even normal fluctuations in the NOx sensor signal downstream of the SCR catalytic converter lead to false detection of NH3 slippage. This would lead to an underestimation of the NH3 storage capacity, and therefore to a false evaluation of the catalytic converter. On the other hand, taking into consideration only large deviations from the normal state increases the reliability of the slippage detection, but in this case the actual NH3 is already very high at the time of slippage detection and falsifies the result of the following emptying test during the underdosing phase, with the SCR efficiency being underestimated. The result in this case, too, can be an incorrect diagnosis. A further disadvantage here is the resulting adverse effect on emissions in particular during the emptying tests, because increased emissions of nitrogen oxides occur as a result of the decreased provision of reducing agent. Furthermore, to condition the SCR catalytic converter for the filling of the NH3 store up to the maximum storage capacity, it is necessary for the NH3 slippage limit to be exceeded. These NH3 emissions are also disadvantageous on account of the environmental impact and the risk to health.
To further improve the monitoring of the SCR catalytic converter, it has already been proposed that the plausibility checking functions be carried out only under certain monitoring conditions. Monitoring processes are limited for example to certain value ranges for one or more of the following variables, which may be modeled or measured, for example with regard to the exhaust-gas mass flow, the exhaust-gas volume flow, the exhaust-gas temperatures, the operating point (speed, injection quantity), the vehicle speed, the ambient pressure, the ambient temperature, NOx, PM, HC, CO or O2 signals, the exhaust-gas recirculation rate, the engine operating mode, the engine status, the engine running time or the engine standstill time. For the diagnosis of the SCR catalytic converter, it is also possible to additionally use the status of the NOx sensors, the actual and nominal filling levels of the reducing agent in the catalytic converter, the regulating error of the NH3 filling level regulator, the status of the reducing agent dosing device, the status/mode of the dosing quantity pilot control, the adaptation factor as a corrective factor for the reducing agent dosing quantity, the status of the dosing quantity adaptation, the status of the diesel particle filter regeneration, the diesel particle filter regeneration demand count, or the status of the HC poisoning. Furthermore, monitoring processes of the SCR catalytic converter may also be carried out under (virtually) steady-state conditions, which may be determined on the basis of one or more of the stated variables.
Despite the restriction of the monitoring conditions, the detection of the occurring NH3 slippage, which indicates that the maximum NH3 storage has been reached at the end of the overdosing phase, remains a problem. The invention is therefore based on the object of improving the NH3 slippage detection as an end point of the overdosing phase, such that the transition from the overdosing phase to the emptying test can be optimized, NH3 emissions can be minimized to the greatest possible extent, and the monitoring is improved overall.